Hydraulic dampers, such as shock absorbers, are used in connection with motor vehicle suspension systems to absorb unwanted vibrations which occur during the operation of the motor vehicle. The unwanted vibrations are dampened by shock absorbers which are generally connected between the sprung portion (i.e. the vehicle body) and the unsprung portion (i.e. the suspension) of the motor vehicle. A piston assembly is located within the compression chamber of the shock absorber and is usually connected to the body of the motor vehicle through a piston rod. The piston assembly includes a valving arrangement that is able to limit the flow of damping fluid within the compression chamber when the shock absorber is compressed or extended. As such, the shock absorber is able to generate a damping force which "smooths" or "dampens" the vibrations transmitted between the suspension and the vehicle body.
A conventional prior art twin tube shock absorber 10 is shown in FIG. 1 and comprises a piston rod assembly 12, a pressure tube assembly 14, and a reserve tube 16. Piston rod assembly 12 is disposed within pressure tube assembly 14 and includes a piston rod 18 having an adaptor 20 at one end which extends out of pressure tube assembly 14 for connection to the motor vehicle. The opposite end of piston rod 18 is attached to a piston valve 22 which is slidably received within pressure tube assembly 14. Pressure tube assembly 14 comprises a pressure tube 24 having a rod guide 26 located at one end and a base valve 28 located at the opposite end. Rod guide 26 slidingly receives piston rod 18 and usually includes a rod bearing 30 disposed between rod guide 26 and piston rod 18 for sealingly engaging piston rod 18. Base valve 28 controls the flow of fluid between a compression chamber 31 defined by pressure tube 24 and a reservoir 32 defined by reserve tube 16. Reservoir 32 coaxially surrounds pressure tube 24 and extends between base valve 28 and rod guide 26. Reserve tube 16 includes a fitting 34 which facilitates the connection of tube 16 to the motor vehicle.
Damping characteristics for shock absorber 10 are controlled by orifices in piston valve 22 and base valve 28 which regulate passage of fluid from one side of piston valve 22 to the other and from compression chamber 31 to reservoir 32. Due to the presence of piston rod 18 on only one side of piston valve 22, the volume of hydraulic fluid which must be displaced on the compression stroke is different from the volume of hydraulic fluid which must be displaced on the rebound stroke. This difference in volume is called the rod volume and it is compensated for by base valve 28 and reservoir 32. The rod volume of hydraulic fluid is throttled out of compression chamber 31 during the compression stroke through base valve 28 into reservoir 32. During the rebound strokes the rod volume of hydraulic fluid enters compression chamber 31 through base valve 28.
The continued movement of piston rod 18 and piston valve 22 back and forth within pressure tube 24 causes the rod volume of oil to be correspondingly throttled into and out of reservoir 32 through base valve 28. Thus, only a portion of the hydraulic fluid in reservoir 32 is effectively utilized. The remainder of hydraulic fluid within reservoir 32 remains relatively static. This quick exchange of hydraulic fluid through base valve 28 and piston valve 22 as well as the friction between piston valve 22 and pressure tube 24 and the friction between piston rod 18 and rod guide 26 generates heat which is undesirable during prolonged operating conditions.
In addition to absorbing the heat generated while providing the damping function for the motor vehicle, shock absorber 10 is also required to operate over a broad range of temperatures ranging from severe cold temperatures of the winter months to the extremely hot temperatures of the summer months. Prior art shock absorbers are manufactured using steel for pressure tube 24 and reserve tube 16. While steel has proven to be an acceptable material for these components, tubes manufactured from aluminum offer the advantages of weight savings as well as improved heat dissipation. If the typical pressure tube 24 were manufactured from steel while reservoir tube 16 were manufactured from aluminum, the difference in their relative axial thermal expansion rates may present problems for the shock absorber when operating over the necessary temperature extremes. Specifically, structural failure may occur under extreme cold temperatures or loss of pressure tube preload and sealing may occur under extreme hot temperatures. Similar problems may occur if pressure tube 24 were manufactured from aluminum and rod guide 26 were manufactured from steel.
Accordingly continued development of shock absorbers with aluminum tubes include the development of methods and components which can eliminate the problems associated with the differing thermal expansion between two different materials.